Re: rand48 replacement

> The important property of determinism is that if I set a seed, and then 
> make an identical set of calls to the random API, the results will be 
> identical every time, so that it's possible to write tests with 
> predictable/repeatable results.

Hmmm… I like my stronger determinism definition more than this one:-)

>> I would work around that by deriving another 128 bit generator instead 
>> of splitmix 64 bit, but that is overkill.
>
> Not really relevant now, but I'm pretty sure that's impossible to do.
> You might try it as an interesting academic exercise, but I believe
> it's a logical impossibility.

Hmmm… I was simply thinking of seeding a new pg_prng_state from the main 
pg_prng_state with some transformation, and then iterate over the second 
PRNG, pretty much like I did with splitmix, but with 128 bits so that your 
#states argument does not apply, i.e. something like:

  /* select in a range with bitmask rejection */
  uint64 pg_prng_u64_range(pg_prng_state *state, uint64 range)
  {
     /* always advance state once */
     uint64 next = xoroshiro128ss(state);
     uint64 val;

     if (range >= 2)
     {
         uint64 mask = mask_u64(range-1);

         val = next & mask;

         if (val >= range)
         {
             /* copy and update current prng state */
             pg_prng_state iterator = *state;

             iterator.s0 ^= next;
             iterator.s1 += UINT64CONST(0x9E3779B97f4A7C15);

             /* iterate till val in [0, range) */
             while ((val = xoroshiro128ss(&iterator) & mask) >= range)
                 ;
         }
     }
     else
         val = 0;

     return val;
  }

The initial pseudo-random sequence is left to proceed, and a new PRNG is 
basically forked for iterating on the mask, if needed.

-- 
Fabien.